Recently, WiMAX (Worldwide Interoprability for Microwave Access, IEEE802.16), one of radio communication standards of the next generation, has attracting attention. WiMAX is a standard expanding speed of communication and mobility of wireless LAN (Local Area Network) (see IEEE 802.16 standard, “Part 16: AIR INTERFACE FOR BROADBAND WIRELESS ACCESS SYSTEMS” August 2007, US. pp. 994-997 (Non-Patent Document 1)).
WiMAX utilizes OFDMA (Orthogonal Frequency Division Multiple Access).
OFDMA system is an expansion of OFDM (Orthogonal Frequency Division Multiplexing) in which data are spread over a plurality of subcarriers, that is, frequencies, so that the data is frequency-multiplexed, to enable allocation among a plurality of users.
Specifically, in OFDMA system, similar to the OFDM system as its base, a plurality of subcarriers are provided in the direction of frequency axis. Different from OFDM, one frame is allocated to a plurality of users in accordance with the frequency (subcarrier) and time (symbol).
The WiMAX communication system utilizes cell configurations and in each cell, a base station (BS) is provided, for example, on a roof top of a building. A mobile station (MS) selects a base station to communicate with, and communicates with the selected base station.
In the uplink of WiMAX, while the base station receives signals from the mobile station, it is expected that signals from a mobile station communicating with a base station of another cell act as interference signals, that is, signals from a mobile station with which communication of the base station of interest is not intended. Such interference signals deteriorate quality of signals received by the base station and, therefore, it is necessary to remove the interference signals in order to improve quality of received signals.
Adaptive array antenna has been known as a method of removing the interference signals. A plurality of antenna elements of the adaptive array antenna receive signals, and the signals received by respective antenna elements are combined with the amplitude and phase controlled, whereby desired signals are generated.
As a result, the antenna comes to have such directivity that, generally, a strong beam is formed in the direction of a desired signal, that is, a signal from a mobile station with which communication of the base station of interest is desired and a weak or null beam is formed in the direction of an interference signal.
In digital control, the amplitude and phase of signals received by each antenna element can be controlled by complex multiplication. The complex here is referred to as a weight.
ZF (Zero Forcing) and MMSE (Minimum Mean Square Error) have been known as methods of calculating the weight. Both methods calculate the weight utilizing signals known on the receiving side.
ZF method uses a weight calculated on the assumption that “combined output signal=transmitted signal,” with noise ignored. Though ZF method exhibits good characteristics in an environment with low noise or weak interference signal, reception quality significantly deteriorates as the interference signal components increase in the received signals.
On the other hand, MMSE uses a weight with which the error power of combined output signal is minimized. The MMSE method has the ability of removing interference signals and, therefore, improves reception quality even when the received signals contain much interference signal components.
Methods of calculation in accordance with MMSE include LMS (Least Mean Square) and RLS (Recursive Least Squared) based on steepest decent method and SMI (Sample Matrix Inversion) as a direct solution using sample values (see Nobuyoshi KIKUMA, “Adaptive Signal Processing Using Array Antenna,” first edition, Kabushiki Kaisha Kagaku Gijutu Shuppan, 1998, pp. 35-66 (Non-Patent Document 2)).
A configuration of deriving a weight using LMS algorithm in OFDM system is disclosed in Japanese Patent Laying-Open No. 2003-174427 (Patent Document 1). Patent Document 1: Japanese Patent Laying-Open No. 2003-174427 Non-Patent Document 1: IEEE 802.16 standard, “Part 16: AIR INTERFACE FOR BROADBAND WIRELESS ACCESS SYSTEMS,” August 2007, U.S.A., pp. 994-997 Non-Patent Document 2: Nobuyoshi KIKUMA, “Adaptive Signal Processing Using Array Antenna,” first edition, Kabushiki Kaisha Kagaku Gijutu Shuppan, 1998, pp. 35-66